Do Ultraluminous X-Ray Sources Really Contain Intermediate-Mass Black Holes?

Abstract

An open question remains whether Ultraluminous X-ray Sources (ULXs) really contain intermediatemass black holes (IMBHs). We carefully investigated the XMM-Newton EPIC spectra of the four ULXs that were claimed to be strong candidates of IMBHs by several authors. We first tried fitting by the standard spectral model of disk blackbody (DBB) + power-law (PL), finding good fits to all of the data , in agreement with others. We, however, found that the PL component dominates the DBB component at ∼ 0.3 to 10 keV. Thus, the black hole parameters derived solely from the minor DBB component are questionable. Next, we tried to fit the same data by the "p-free disk model" without the PL component, assuming the effective temperature profile of T eff ∝ r −p where r is the disk radius. Interestingly, in spite of one less free model parameters, we obtained similarly good fits with much higher innermost disk temperatures, 1.8 < kT in < 3.2 keV. More importantly, we obtained p ∼ 0.5, just the value predicted by the slim (supercritical) disk theory, rather than p = 0.75 that is expected from the standard disk model. The estimated black hole masses from the p-free disk model are much smaller; M < ∼ 40M ⊙ . Furthermore, we applied a more sophisticated slim disk model by Kawaguchi (2003, ApJ, 593,69), and obtained good fits with roughly consistent black hole masses. We thus conclude that the central engines of these ULXs are super-critical accretion flows to stellar-mass black holes.

Spectral state transitions of the Ultraluminous X-ray Source IC 342 X-1

“…This model has also been used to fit other ULX spectra (e.g. Foschini et al 2006;Vierdayanti et al 2006;Godet et al 2012;Bachetti et al 2013). The parameters of the slim disc fit are listed in Table 5.…”

X-ray observations of ultraluminous X-ray sources

“…This would manifest itself as a change in the model disc profile, T (r) ∝ r −p , where standard discs have p = 0.75, and slim discs p = 0.5. Recent work where a variable disc profile model is fit to ULX spectra does indeed show values of p ∼ 0.6, suggestive of slim discs (Vierdayanti et al 2006;Mizuno et al 2007). A second idea, put forward by Goncalves & Soria (2006), draws from observations of AGNs with outflows.…”

THE ULTRALUMINOUS X-RAY SOURCES NGC 1313 X-1 AND X-2: A BROADBAND STUDY WITHNuSTARANDXMM-Newton

“…But the temperature-luminosity relation for this component does not match that expected in standard accretion disks in the soft state, where the disks extend to the innermost stable circular orbit (see, e.g., Kajava & Poutanen 2009;Feng & Soria 2011 for a review). This relation can be partially recovered in some cases by assuming a constant absorption column between the observations or using non-standard disk models (Vierdayanti et al 2006). Also, the cutoff is at much lower temperature than is expected in standard BH hard states (Done et al 2007).…”